1. Field of the Invention
The present invention relates to semiconductor laser devices of the mold-type in which a laser diode element is encapsulated in a sealing resin layer, and more particularly to a semiconductor laser device that experiences minimal displacement of its light emitting point during continuously operation and/or during changing ambient conditions.
2. Discussion of the Prior Art
A semiconductor laser device of the can-type as shown in FIGS. 6, and 7A and 7B is known in the art. As shown in FIG. 6, a laser diode element 1 is soldered to a radiator 62 supported on a stem 61. A cap 63 with a glass window 64 is soldered to the stem 61. In the semiconductor laser device shown in FIGS. 7A and 7B, a laser diode element 1 is mounted to a submount provided by a photo diode 23, which is then mounted on a radiator 62 extending upwardly from the stem 61. The photo diode 23 submount also serves as a radiator plate. The laser diode element 1 is covered with a cap 63 also fastened to the stem 61. The combination of the radiator 62 and the photo diode 23 are positioned so that the laser diode element 1 As located at the center of the glass window 54 An cap 53, as viewed from the front; the center being designated by an intersection point 27 of a horizontal center line 25 (at a right angle to the major surface of the radiator 62, or an X-axis direction) and a vertical center line 26 (parallel to the major surface of the radiator 62, or a Y-axis direction).
Another semiconductor laser device of a resin sealing mold-type has been developed. This type of laser device is less expensive and can be shaped more flexibly than the can-type laser device disclosed above. The mold-type laser device is described in detail in Published Unexamined Japanese Patent Application No. Hei. 2-125687. In this device, as shown in FIG. 8 herein, a laser diode element 1 is mounted on a submount 23 and then sealed in a sealing resin layer 11 comprised of transparent epoxy resin, for example. The element 1 is electrically powered through lead frames 20 and a gold wire 21. The semiconductor laser device of the mold-type has been known as a light emitting device of low light density per unit area, analogous to an LED.
The laser device of the mold-type is advantageous because of low manufacturing cost and the resin sealing layer can be of a wide variety of shapes. Additionally, the laser diode element can be used in high light density applications without any characteristic deterioration owing to light damage if an end face destruction preventing layer is used. A semiconductor laser device incorporating an end-face destruction preventing layer is described in detail in commonly assigned U.S. Application Ser. No. 07/788,601, filed Nov. 6, 1991, U.S. Pat. No. 5,355,385.
One example of the laser device disclosed in patent application Ser. No. 07/788,601 will be described with reference to FIGS. 9 through 12. The laser diode element 1 is illustrated as having a DH (double heterodyne Junction) structure. As shown, an n-type clad layer 3 made of AlgaAs, an active layer 4, a p-type clad layer 5, and a p-type cap layer 6 made of GaAs are layered on an n-type GaAs substrate 2. An electrode 7 is selectively formed on the obverse side of the laser diode element in an opened portion of the p-type cap layer 6. An electrode 8 is formed on the rear side of the substrate 2. End-face destruction preventing layers 10 (FIG. 10) are respectively formed on the light emitting end faces of the laser diode element and are irradiated with laser beams. The end-face destruction preventing layers 10 are made of organic resin, which exhibits high heat resistance and low absorption coefficient in the wavelength region of the emitted laser beams.
As best illustrated in FIG. 11, the laser diode element 1 is mounted on a photo diode 23 serving also as a submount layer and a radiator plate. The photo diode 23 is mounted on the top end portion of the center lead frame of laterally arrayed lead frames 20. The photodiode 23 and the p-type cap layer 6 are connected to the respective lead frames 20 by bonding wires, for example, gold wires (not shown). The laser diode element 1, connected to those lead frames 20, is enclosed by resin 11, such as transparent epoxy resin, in a sealing manner. The semiconductor laser device of the mold-type, which includes the laser diode element 1 having the end-face destruction preventing layers 10, is low in cost and good in endurance.
In the semiconductor laser device of the mold-type, as shown in FIGS. 12A and 12B, the laser diode element 1 is positioned at or near the center the sealing resin layer 11, which is designated by intersection point 27 of the X axis 25 and the Y axis 26, as in the semiconductor laser device of the can-type. The center 28 of the lead frame 20 is displaced by distance .DELTA.X.sub.off (or offset 29), from the intersection point 27 of the sealing resin layer 11, because of the total thickness of the laser diode element 1, the photo diode 23 and the lead frame 20.
Those semiconductor laser devices are assembled into various types of optical systems. A typical example of the optical system is a pick-up device for an optical disc as schematically illustrated in FIG. 13A.
In the pick-up device illustrated, a laser beam emitted from the laser diode element 1 passes through a diffraction grating 51, turned 90.degree. by a half mirror 52, and focused on the surface of a disc 54 through an objective lens 53. The laser beam is separated into a main beam and a subbeam by the diffraction grating 51. The subbeam is used for tracking servo purposes. The laser beam, reflected by the disc, 54 passes through the objective lens 53 and the half mirror 52 again and is projected onto the light sensor 55. The sensor transforms the received laser beam into a corresponding electrical signal.
The light sensor 55 consists of six photo diodes A to F, as shown in FIG. 13B. The main beam is incident on the quartered diodes A to D. The laser beam experiences astigmatism as it passes through the half mirror 52, and the shape of the main beam changes as indicated by the dotted lines in FIG. 13B, depending on the location of the photo diodes.
The focusing servo mechanism positions the objective lens 53 so that (A+C)-(B+D)-0 where A, B, C and D are outputs of the photo diodes A, B, C and D. The "gravity center of beam on the photo diodes", to be discussed later, is defined as EQU [X, Y]=[{(A+B)-(C+D)}/(A+B+C+D),{(A+D)-(B+C)}/(A+B+C+D)]
In the semiconductor laser device of the mold-type, it has been discovered that the light emitting point, i.e., the origin of the emitted laser beam, shifts location when the laser device is continuously operated and/or ambient conditions change. Shifting or displacement of the light emitting point is graphically represented in FIG. 14. To plot the graph of the figure, a mold-type semiconductor laser device was operated at room temperature while being fed with an operating current of 50 mA. In the graph, the abscissa represents time in minutes, and the ordinates represents the extent of displacement in the X-axis direction (normal to the lead frame surface. As seen from the graph, after about two minutes of laser emission, the light emitting point was displaced 0.5 .mu.m in the-X-axis direction (i.e., toward the lead frame 20). After the device was turned off for about two minutes, the light emitting point returned to the original center point (moved in the+X-axis direction, or toward the laser diode). When the semiconductor laser device is assembled into a pick-up device, the extent of displacement of the gravity center of the beam on the photo diodes depends on the duration of continuous laser device operation and/or changes in ambient temperature.
The semiconductor laser device was operated at an output power of 3 mW at ambient temperatures from -10.degree. C. to 60.degree. C. The gravity center of beam on the photo diodes was found to be displaced 10 .mu.m or more. Minimization of the displacement of the light emitting point is essential in order that the semiconductor laser devices of the mold-type, which has many advantages, acquire the excellent performance achieved by can-type semiconductor laser devices.
Turning to FIG. 15, there is shown a simulation model of the influence of light emitting point displacement when the laser device operates in a pick-up device. In the simulation, a one-dimensional optical system was utilized, in which a laser beam from a laser diode LD1, located on the left side, passes through a convex lens 79, separated from the diode LD1 by a distance d1, and the laser beam, reflected by a disc 73, then passes through another convex lens 80 to be focused on a photo diode (PD) 69. The one-dimensional system was treated as a double Fourier transform optical system. A light intensity distribution U2 on the convex lens 79, a light intensity distribution U3 on the disc 73, a complex amplitude U4 on the convex lens 80, and a light intensity distribution U5+.DELTA. were calculated using Fresnel's diffraction formula, with the assumption that the light intensity distribution U1 on the laser diode element 1 is rectangular in shape, is centered at .DELTA.X and has a width of 2 .mu.m. The .DELTA.X corresponds to the extent of displacement of the light emitting point of the laser diode. The results of the calculations using the parameters in Table 1 are shown in FIGS. 16A-16J.
FIGS. 16F-16J are graphical representations of the light distributions U1, U2, U3, U4 and U5+.DELTA.X=0, and FIGS. 16A-16E is a graphical representations of the same when .DELTA.X=1 .mu.m. A beam spot gravity center on the photo diode can be calculated using the light intensity distribution (U5+.DELTA.) on the photo diode. When the displacement quantity .DELTA.X of the light emitting point was 1 .mu.m, a shift of the beam spot gravity point, when calculated, was 7.9 .mu.m. Similar calculations were repeated for other quantities of the displacement. The results of the calculation showed the relationship between the quantity of displacement of the light emitting point and the quantity of shift of the beam spot gravity center on the photo diode, as shown in FIG. 17. Thus, in an pick-up optical system, 1 .mu.m displacement of the light emitting point produces a 7.9 times larger shift in the beam spot gravity center on the surface of the photo diode. This value is defined as a coupling magnification M between the laser diode element and the photo diode. The coupling magnification M is determined by the magnification of the lens system and the distance of the optical path between the light emitting point of the laser diode element 1 and the signal detection, divided photo diode. Accordingly, the coupling magnification M differs with the construction of the optical pick-up device.
TABLE 1 ______________________________________ .DELTA.x 1.0 .mu.m d1 25.0 .mu.m f 3.9 .mu.m d2 4.6 .mu.m lens diameter 4.0 .mu.m .DELTA. 0.3 .mu.m ______________________________________ Note: For the symbols in the table, refer to FIG. 15.?
In the conventional semiconductor laser device, the extent of the shift of the beam spot on the photo diode 69 can exceed a tolerable value. Accordingly, a mechanism to adjust the displacement of the light emitting point or a mechanism to follow the shift of the beam spot gravity center is required for the conventional semiconductor laser device.
Thus, the semiconductor laser device of the mold-type is advantageous in that it is inexpensive and can be flexibly shaped. However, when it is utilized in an application requiring good light emitting point stability, an additional adjusting mechanism is required. This negates the advantages of laser devices of the mold-type.